A single-frequency network (SFN) is a broadcast network where multiple transmitters simultaneously transmit the same signal over the same frequency channel. Some examples of SFNs include Digital Video Broadcasting-Terrestrial (DVB-T) and Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) systems. DVB-T is a coded orthogonal frequency division multiplexing modulation (COFDM) system. In a DVB-T system, a number of time-shifted versions of the same transmitted signal are received by the DVB-T receiver. The distribution of path delays between the signals is known as the delay spread of the channel. The delay spread causes the transfer function of the channel to vary over frequency which results in inter-symbol interference (ISI) and frequency selective fading.
In a DVB-T or ISDB-T system, a cyclic prefix is inserted as a guard interval (with a length of ¼, ⅛, 1/16, or 1/32 of one symbol) to combat the ISI caused by channel delay spread. An ISI-free transmission may be guaranteed when the channel length is shorter than the guard interval. Increasing the length of the guard interval, however, may reduce the channel efficiency.
Pilots are also transmitted, on selected carriers, to equalize the received signal, estimate the channel response, determine the signal to noise ratio (SNR), and to assist in timing and synchronization. There are two types of pilots that are commonly used in an SFN; continuous pilots and scatter pilots. Continuous pilots are transmitted in every symbol whereas scattered pilots are repeated periodically, such as every four symbols. The pilot carriers are identified by carrier indexes. An example pilot structure of a DVB-T system is shown in FIG. 1. Pilots are transmitted using binary phase-shift keying (BPSK). The pilot carriers have a boosted power level of 16/9, compared to QPSK/16 QAM/64 QAM with power level of 1/1 for data carriers. The power boost assures that the channel response of the pilot carriers (HP) can be reliably estimated.
FIG. 1 shows an example pilot structure of an OFDM DVB-T system. The bit stream is split into parallel data streams, each transferred over its own carrier using BPSK. The modulated carriers may be summed to form an OFDM signal. A bitstream is transferred over a communication channel using a sequence of OFDM symbols. As shown in FIG. 1, in one symbol, there is one pilot inserted every twelve carriers. The scattered pilot pattern repeats every four symbols. Combining the pilots from four symbols gives one pilot every three carriers. A channel estimate (ĤP) that is generated based on the pilots of a scattered pilot pattern is a downsampled-by-three version of the overall channel frequency response H.
In DVB-T, the continuous pilots are a sub-set of scattered pilots. Both continuous/scattered pilots only use a portion of all the carriers in one symbol. The channel response on these pilot carriers is first estimated. The channel response may be estimated for the data carriers based on any known algorithm, including least square (LS), minimum mean-square error (MMSE) or Modified MMSE. The estimation can be performed once per symbol.
FIG. 2 is a flow diagram of a method (200) to estimate the channel frequency response of an OFDM system. A handset receives the pilots over pilot carriers with boosted power levels 205. The handset then determines the channel response HP of the data carriers that are transmitted in between the transmission of the pilot carriers 210. The channel response HP may be determined using interpolation based on the channel estimate ĤP. Next, the handset performs an inverse fast Fourier transform (IFFT) on the channel estimate ĤP, to generate a resolution-reduced channel impulse response (CIR) ĥ (215). The resolution-reduced CIR ĥ is used to adjust the symbol timing, which refers to the point where individual OFDM symbols start and end (220). A fast Fourier process is performed, wherein the adjusted symbol timing is used to define the fast Fourier transform window (225). This method, however, may result in aliasing. Because the channel estimate ĤP is the downsampled-by-three version of the channel frequency response H, the resolution-reduced CIR ĥ will be repeated at Tu/3 interval, where Tu is the time span of one OFDM symbol. Therefore, any channel longer than Tu/6 will cause aliasing, as shown in FIG. 3.
FIG. 3 is a graph showing an aliasing problem associated with a long channel. Instead of a post-cursive channel with length of Tu/4, the resolution-reduced CIR ĥ becomes a pre-cursive channel with length of Tu/12 because of the use of the window [−Tu/6, Tu/6], which can result in aliasing. Aliasing can affect both the symbol timing and channel estimation, which thereby causes a demodulator malfunction.
The aliasing problem can be partially resolved by designing a system that weighs the post-cursive channel more heavily. However, this design only improves the aliasing problem in a channel with a constrained channel length and without any outside guard echoes. Current solutions focus on generating a channel estimate based only on the continuous/scattered pilot signal. However, if the channel impulse response is too long, the continuous/scattered pilots are not transmitted frequently enough to recover all the channel information.